When demands are very high, direct drives, e.g. linear motors, are employed for eliminating play and elasticity of a mechanical transmission. This high level of complexity is evident, in particular, in linear drives and slowly rotating direct drives. Besides the high cost, said drives have a great volume and poor efficiency.
In many applications for high-precision rotational movements, electromechanical drives are employed today. For providing the required small rotational speeds for the working machine, motor/gear combinations having low-play planetary gear units adapted for the required accuracy are employed.
Where the remaining inaccuracy of mechanics is too great due to gear backlash and elasticity of belts or chains, direct drives are employed, as described above. For realizing mechanically precise drives, for example, the following measures are applied today for improving accuracy.                Low-play gear units ensure slight angle errors by means of narrow tolerance toothing elements and play-free clutches, e.g. multiple-disk clutches, metal bellow clutches.        Torsion-proof transmission elements and gear units, e.g. planetary gear units, thick-walled housings, enhance the achievable reinforcements and also realize slight angle errors, where necessary.        Direct drives, e.g. slowly rotating synchronous motors, avoid errors caused by mechanical transmission.        
What today's solutions have in common is that the drive is to be adapted for full power and the required precision. This renders the drives expensive. The described measures, in particular in connection with high power, involve a great deal of expense. The process-safe realization in production constitutes a further problem, for example, the selection of precision toothing elements for a low-play gear unit is possible with restrictions only in connection with high torques.
In the case of speed-regulated high-power drives with simultaneously high demands on accuracy or dynamics, reducing the very high level of complexity of the power electronics (high clock frequency), the motor (high precision) and mechanical transmission (low-play transmission elements) is desirable.